So, you being guys and him being a guy, can anyone explain to me why it is that his undies are too big, in that first before pic as if they've been used when he's been bigger and now they're stretched?

I had underwear like that once that kind of pulled off my waist. They were made with a little more firm type of blend/material, and during periods of normal turgidity (no shrinkage going on) they pulled away from my waist a bit.